The motors for those antennae are pretty expensive, the trick with mine was: get it shimmied up to full mast and disconnect it so that it won't come down... the bad part of that is, as you may have guessed, it won't come down. I actually have an 85 300D that is possibly meeting its maker this summer and it may be putting its sunroof motor on the auction block...

As for the sunroof, you may try to "clean" (just wipe clean) the track, then re grease. I used a combination of a light oil (3-in-1) and petroleum jelly. Before I did this, it groaned loudly when opening and closing. Now it's silent and moves freely. You my also take a look at a grease tube that is hooked to the back of the sliding roof's motor and runes behind the plastic trunk panel on the drivers side. It should just slide up and off the motor. You may check to see that it is still straight and isn't dented or pinched anywhere but at the end. My antenna mast was also being super funny with me, but some one before me installed a new motor in the car. I found that they are picky with their ground and need a perfect ground connection. Also, my connectors were a little gummed up. Now it works ok...has not problem going up, but going down doesn't happen too often. lol Hope this helps! Good luck!

My 82 240D had a switch that would select partial or full extension of the antenna. If it is jamming, then replace the mast before the motor is damaged. It is easy to do and the mast is around $30. If I look at a used Mercedes, and the antenna is rigged up all the way or some POS rubber replacement has been installed, I immediately suspect the care of the vehicle.

The standard action of the antenna is to go half way when the radio is turned on. If it does not extend fully, something can be wrong inside the assembly. Could be the mast, or the gearing inside (plastics breaking).